Nitrogen generation

ABSTRACT

Air is separated in a single fractionation column ( 116 ) to produce a top nitrogen gaseous fraction ( 142 ) and a bottom liquid fraction ( 120 ) containing less than (80) mole percent of oxygen. A liquid nitrogen product ( 136 ) is also produced. The necessary refrigeration is created by expansion with the performance of external work of firstly a stream of compressed air in an expansion turbine ( 114 ) and of secondly a stream of vaporised bottom fraction in an expansion turbine ( 128 ). At least part of the feed to the column ( 116 ) comes from the turbine ( 114 ). The outlet pressure of the turbine ( 114 ) is essentially the gaseous nitrogen product pressure. A double fractionation column may be used instead of the single fractionation column ( 116 ).

[0001] This invention relates to nitrogen generation. More particularly, it relates to a method of and apparatus for generating a nitrogen product at an elevated pressure.

[0002] The separation of air is a particularly important process industrially because the products of the separation, primarily oxygen and nitrogen, have abandoned industrial and medical uses. A particularly common method of separating air is by fractionation at cryogenic temperatures. The configuration of a cryogenic air separation plant depends on the purity of the product. Commonly, oxygen and nitrogen products, both with a purity greater than 99% by volume are required. The configuration of an air separation plant is also influenced by whether a liquid oxygen or liquid nitrogen product is required. Air separation plants producing relatively pure products are generally particularly energy intensive, particularly if some of the products are required in liquid state.

[0003] If only a nitrogen product is required, then it is generally economically justified not to produce a pure oxygen product in addition to the nitrogen product, but instead to separate a relatively impure oxygen-enriched waste air products a result, simpler plant designs come into their own enabling a given quantity of nitrogen product to be produced at a lower capital cost and operating cost than in a plant producing pure oxygen and pure nitrogen products. Accordingly, so called nitrogen generators are typically of a very different design from air separation plant producing both pure nitrogen and pure oxygen products.

[0004] If desired, a nitrogen generator may employ but a single fractional distillation or rectification column (“fractionation column”).

[0005] Typically, the air to be separated is compressed and purified. The resulting compressed, purified air is passed into the single rectification or fractionation column. There it is separated into a top-vaporous nitrogen fraction and a bottom liquid oxygen-enriched air fraction. A stream of the oxygen-enriched liquid air fraction is withdrawn from the bottom of the single rectification column, is expanded through a throttling valve, and is employed to condense the top nitrogen fraction by indirect heat. This indirect heat exchange typically causes oxygen enriched liquid air stream to evaporate. The resulting vaporised oxygen-enriched air stream is expanded in a turbine with a performance of external work. The turbine expansion typically meets all of the refrigeration requirements of the air separation process. The nitrogen product is taken in gaseous state from the top of this single fractionation column.

[0006] A drawback of the above-described process is that the recovery of nitrogen product from the incoming air is not very high as a result of venting from the process the stream of the bottom fraction exiting the expansion turbine. EP-A-412 793 relates to an improvement in which not all of the bottom fraction is expanded in a turbine. Instead, a part of the vapour exiting the nitrogen condenser is recompressed and returned to the fractionation column. EP-A-0773 417 relates to a further improvement in which the stream which is recompressed is taken not from the bottom liquid fraction of the single column but instead from an intermediate region thereof. Other single column nitrogen generators are disclosed in U.S. Pat. No. 5,611,218, U.S. Pat. No. 5,704,229, and DE-A-198 02 610.

[0007] The prior nitrogen generators of the kind discussed above are all able to produce an elevated pressure nitrogen product without requiring a product compressor. The single distillation column is arranged to operate at the pressure to which the incoming air is compressed. A drawback of such nitrogen generators is, however, that they employ but a single expansion turbine and are therefore not able to produce a liquid nitrogen product in any substantial quantity. Sometimes it is required to produce as much as 10% of the nitrogen product in liquid state. In this case, it is necessary to employ a second expansion turbine. EP-A-0 932 004 (97A639) discloses feeding a part of the air that has been compressed to the operating pressure of the single fractionation column to a second expansion turbine and expanding the feed air to approximately atmospheric pressure and venting the expanded feed air from the generator. A disadvantage of this arrangement is that it negates the beneficial effect that the recycle of gas to the single fractional column has on the recovery of nitrogen product.

[0008] Nitrogen generators employing a double fractional column are also known. For example, U.S. Pat. No. 4,717,410 discloses double column nitrogen generator in which all the nitrogen product is taken from the higher pressure column. In order to enable this to be achieved, liquid nitrogen is pumped from the top of the lower pressure column to the top of the higher pressure column. In addition, liquid nitrogen is added from an external source. Thus, far from being a net producer of liquid nitrogen the nitrogen generator according to U.S. Pat. No. 4,717,410, actually consumes liquid nitrogen.

[0009] It is an aim of the present invention to provide a method of (and nitrogen generator for) generating a nitrogen product at elevated pressure which enables a liquid nitrogen product to be co-generated without compromising nitrogen recovery.

[0010] According to the present invention there is provided a method of generating nitrogen, wherein compressed air is separated at elevated pressure by fractionation to produce a product gaseous nitrogen fraction and a product oxygen-enriched liquid fraction containing less than 80 mole percent of oxygen, a gaseous nitrogen product is taken at elevated pressure from the gaseous nitrogen product fraction, a stream of the product oxygen-enriched liquid fraction is vaporised by indirect heat exchange with condensing nitrogen separated in the fractionation, a liquid nitrogen product is generated with the gaseous nitrogen product, by condensing nitrogen separated in the fractionation and refrigeration for the method is created (i) by expanding a stream of compressed air in a first expansion turbine with the performance of external work and (ii) by expanding at least part of the vaporised stream of the product oxygen-enriched fraction in a second expansion turbine with the performance of external work, characterised in that the said stream of compressed air enters the first expansion turbine at a pressure substantially in excess of the pressure at which the gaseous nitrogen product is taken but leaves the first expansion turbine at essentially the gaseous nitrogen product pressure, and in that at least part of the air separated in the said fractionation column is supplied from the first expansion turbine.

[0011] The invention also provides a nitrogen generator for performing the above—defined method, comprising a fractionation column operable to produce an elevated pressure gaseous nitrogen product and an oxygen-enriched air product containing less than 80 mole % of oxygen, the nitrogen generator also being able to co-generate a liquid nitrogen product, a first expansion turbine for expanding with the performance of external work a first stream of compressed air, a nitrogen condenser for condensing nitrogen separated in the fractionation column by indirect heat exchange with a vaporising stream of the product oxygen-enriched liquid air, and a second expansion turbine for expanding a stream of the vaporised product oxygen-enriched air with the performance of external work, characterised in that the outlet of the first expansion turbine communicates with the fractionation column.

[0012] By arranging for the first expansion turbine to discharge into the fractionation column, it is possible to produce up to at least 10% of the nitrogen product in liquid state without compromising the nitrogen recovery of the method and nitrogen generator according to the invention. Another important advantage of the method and nitrogen generator according to the invention is that the amount of refrigeration produced and hence the amount of liquid nitrogen product co-generated with the elevated pressure gaseous nitrogen product can be varied by adjusting the pressure of the air flowing to the inlet of the first expansion turbine. Such adjustment can readily be effected without altering the outlet pressure of the first turbine and, therefore, without altering the pressure at which the elevated pressure gaseous nitrogen product is produced. Such a facility is not available in the prior nitrogen generators employing but a single expansion turbine because there is no facility to adjust its inlet pressure without substantially effecting the other operating parameters of the generator. Similarly, the nitrogen generator according to EP-A-0 932 004 ties the inlet pressure of its air expansion turbine to the operating pressure of its fractionation column, so that any change in the inlet pressure to the air expansion turbine will effect the pressure at which the gaseous nitrogen product is produced.

[0013] The method according to the present invention may be applied equally to a single column nitrogen generator or a double column nitrogen generator. In the example of a single column nitrogen generator, a stream of a liquid mixture of nitrogen and oxygen is preferably withdrawn from the bottom, or more preferably an intermediate mass exchange region of the single rectification column, is reduced in pressure, is heat exchanged with condensing nitrogen separated in the single rectification column, and is vaporised thereby, and a stream of the resulting vapour is compressed in a recycle compressor and returned preferably to a bottom mass exchange region of the single rectification column. The returning recompressed vapour may be mixed with the air that is fed to the single rectification column for separation, but this is not preferred.

[0014] In the example of a double fractionation column the stream that is expanded in the second expansion turbine is vaporised by heat exchange with condensing nitrogen that has been separated in the lower pressure fractionation column. Preferably, in the example of the double fractionation column, all the elevated pressure gaseous nitrogen product is taken from the higher pressure column. In order to facilitate this, liquid nitrogen separated in the lower pressure rectification column is preferably pumped to the higher pressure rectification column. This helps to enhance the liquid nitrogen reflux in the higher pressure rectification column and reduces its reliance on the condenser placing the top of the higher pressure column in heat exchange relationship with the bottom of the lower pressure column for producing such reflux.

[0015] Preferably, at least 85% by volume of the gaseous air to be separated in the fractionation column flows thereto from the first expansion turbine. A separate stream of liquefied air may also be introduced into the fractionation column. A further possibility is not to have a separate introduction of liquid air into the fractionation column, but instead to arrange to operate the method according to the invention such that the stream leaving the first expansion turbine and/or the recompressed stream of a mixture of oxygen and nitrogen enter the fractionation column in the presence of a small amount of liquid air.

[0016] The stream of air leaving the first expansion turbine is preferably cooled in heat exchange with the elevated pressure product nitrogen stream and with the oxygen enriched air product stream, upstream of its introduction into the fractionation column.

[0017] The stream of product oxygen-enriched air leaving the second expansion turbine is preferably warmed in heat exchange with the compressed air stream flowing to the first expansion turbine.

[0018] The patent oxygen-enriched air stream is typically vented to the atmosphere as a waste stream. The term “product” as used herein encompasses such a waste stream.

[0019] If desired, the method and nitrogen generator according to the invention can produce a gaseous nitrogen product containing less than one part per billion of oxygen impurity.

[0020] Methods and nitrogen generators according to the invention will now be further exemplified with reference to the accompanying drawings, in which

[0021]FIG. 1 is a process flow diagram of a single fractionation column nitrogen generator according to the invention, and

[0022]FIG. 2 is a schematic flow process diagram of a double fractionation column nitrogen generator according to the invention.

[0023] The drawings are not to scale.

[0024] Referring to FIG. 1, air is compressed in a compressor 102. The compressor 102 has a plurality of compression stages (not shown) and is provided with intercoolers (not shown) so as to remove heat of compression from the air as it flows from one compression stage to the next. The resulting compressed air flows through an aftercooler 104 to remove the heat of compression imparted to the air in the final compression stage of the compressor 102. The resulting air flows from the aftercooler 104 at a temperature approximately equal to or a little below ambient temperature to a purification unit 106. The primary purposes of the purification unit 106 are to remove essentially all water vapour and carbon dioxide from the air and also to remove hydrogen and carbon monoxide impurities. The removal of water vapour and carbon dioxide impurities may be defective by adsorption. Pressure swing and temperature swing air purification processes are both well known in the art and need not be described further herein. Hydrogen and carbon monoxide impurities may be removed catalytically by the method according to EP-A-0 438 282 which is incorporated herein by way of reference. The resulting purified compressed air stream flows to a main heat exchanger 108.

[0025] The purified air flow enters the main heat exchanger 108 through its warm end 110. It is cooled by passage through the main heat exchanger 108. Approximately 90% of the air flow is withdrawn from the main heat exchanger at an intermediate region thereof and flows to a first turbo-expander or expansion turbine 114 in which it is expanded with the performance of external work to a chosen temperature or pressure. The chosen temperature is a cryogenic temperature that one above that at which the air is to be separated by fractionations. The chosen pressure is a little above that at which the air is to be fractionated. The resulting expanded air flow is reintroduced into the main heat exchanger 108 and is further cooled therein. The further cooled expanded air flow is withdrawn from the cold end 112 of the main heat exchanger and is introduced into an intermediate mass exchange region of a fractionation or rectification column 116 without any further expansion or compression.

[0026] That part of the purified air flow which is not taken from the main heat exchanger at 108 for expansion in the first turbo-expander 114 continues its passage through the main heat exchanger, exiting it from its cold end 112. This remaining air stream preferably exits the cold end 112 at the main heat exchanger 108 in liquid state. The resulting liquid air stream passes through an expansion or throttling valve 118 and is thereby expanded to the operating pressure of the fractionation column 116. It is introduced into the column 116 either at the same level as the air from the first turbo-expander 114 or at a level thereabove.

[0027] The air is separated in the fractionation column 116 into a bottom product liquid fraction enriched in oxygen (typically containing in the order of 45 to 60 mole % of oxygen) and a top product vaporous nitrogen fraction. The fractionation column 116 contains liquid-vapour contact devices (not shown) for effecting intimate contact between ascending vapour and descending liquid. Typically these devices take the form of structured packing. A stream of the oxygen-enriched liquid fraction is withdrawn through an outlet 120 from the bottom of the fractionation column 116. The oxygen-enriched liquid air stream is sub-cooled in a heat exchanger 122. The resulting sub-cooled oxygen-enriched liquid air stream is expanded by passage through a throttling or expansion valve 124. The resulting expanded oxygen-enriched liquid air stream is passed to a first condenser 126 associated with the top of the fractionation column 116. The expanded oxygen-enriched liquid air stream provides cooling for the first condenser 126 which is employed to condense a part of the vaporous nitrogen fraction separated in the fractionation column 116. Part of the resulting liquid nitrogen is returned to the top of the fractionation column 116 as reflux. The oxygen-enriched liquid air stream is at least partially vaporised in the first condenser 126. The resulting at least partially vaporised stream is introduced into the main heat exchanger 108 through its cold end 112. The vaporised oxygen enriched air stream is warmed in the main heat exchanger and is withdrawn from an intermediate region thereof. The warmed oxygen-enriched air stream is expanded with the performance of external work in a second expansion turbine or turbo-expander 128. The oxygen enriched air stream leaves the turbo-expander 128 at approximately the cold end temperature of the main heat exchanger 108 and a pressure a little above atmospheric pressure. This stream is returned through the main heat exchanger 108 from its cold end 112 to its warm end 110. On leaving the warm end 110 it may be vented to the atmosphere but is referred to herein as a “product” because it is discharged from the nitrogen generator. The stream of oxygen-enriched liquid air withdrawn from the bottom of the fractionation column 116 through the outlet 120 provides only a part of the cooling necessary for the condensation of nitrogen at the top of the fractionation column. A second stream of liquid air is withdrawn from a chosen region of the rectification column 116 through an outlet 130. The position of the outlet 130 is chosen such that the liquid air stream withdrawn therethrough has a higher mole fraction of oxygen than normal atmospheric air but a lower mole fraction of oxygen than the oxygen-enriched liquid air stream withdrawn through the outlet 120. The stream withdrawn through the outlet 130 is sub-cooled by the passage through the heat exchanger 122. The sub-cooled liquid air stream is reduced in pressure by expansion through a further expansion or throttling valve 132. The thus expanded liquid air stream is employed to provide cooling to a second nitrogen condenser 134 associated with the top of the fractionation column 116. Further nitrogen vapour separated in the fractionation column is thereby condensed in the second condenser 134 and a part of it is returned to the top of the fractionation column 116 as reflux. A part of the liquid nitrogen condensate is withdrawn through an outlet 136 to be collected as liquid nitrogen product. The liquid air stream is vaporised in the second condenser 134 and the resulting vapour stream flows to a cold compressor 138 in which it is recompressed to the operating pressure of the fractionation column 116. The resulting compressed vapour stream is introduced into the main heat exchanger 108 at an intermediate region thereof and is cooled therein to a temperature suitable for a separation in the fractionation column 116. The resulting cooled recompressed air stream exits the main heat exchanger 108 at its cold end 112 and is returned to the fractionation column 116 through an inlet 140 situated at a bottom region of the fractionation columns 116.

[0028] A gaseous nitrogen product stream is withdrawn from the top of the fractionation column 116 and flows along a conduit 142 through the heat exchanger 122 and the main heat exchanger 108 from its cold end 112 to its warm end 110 (thereby providing cooling for the heat exchangers 108 and 122). The resulting warmed nitrogen stream may be taken from the warm end 110 of the main heat exchanger 108 as a product gaseous nitrogen stream. The purity of this nitrogen product and of the liquid nitrogen product which is taken through the product nitrogen outlet 136 depends on the number of theoretical trays and hence the actual height of packing in the fractionation column 116. If very high purity nitrogen is required, sufficient theoretical trays can be provided to ensure that the nitrogen product contains less than one part per 1000 million by volume of oxygen.

[0029] The apparatus shown in FIG. 1 is intended to be operated continuously. The fractionation column 116 operates with a substantially constant pressure at its top, typically in the order of 11 bar. Similarly, the vaporising pressure on the vaporising side of the first condenser 126 (as well as that on the vaporising side of the second condenser 134) is essentially constant, giving a constant inlet pressure to the second turbo expander 128 typically in the order of 5 bar. The second turbo expander 128 therefore produces a constant amount of refrigeration per unit time. The total amount of refrigeration per unit time required by the method according to the invention depends on the proportion of nitrogen product which is produced in liquid state. The greater this proportion, the more refrigeration is needed. An advantage of the method according to the invention is that essentially the same plant design can be employed for different proportions of liquid nitrogen product. This flexibility can be obtained by virtue of the fact that the inlet pressure of the first turbo expander 114 can be set without substantially affecting other parameters in the method according to the invention. In a typical example in which, say, 10% of the nitrogen product is produced in liquid state the first turbo-expander 114 may have an inlet pressure in the order of 16 bar. The main air compressor 102 is therefore arranged so as to compress the incoming air to a pressure a little above 16 bar. A further advantage of the method according to the invention is that the apparatus shown in FIG. 1 may readily be adjusted so as to change the proportion of liquid nitrogen product that is produced. This can be done by changing the pressure to which the air compressor raises the incoming air and hence the inlet pressure of the first turbo-expander 114. To this end, the main air compressor 102 may have adjustable guide vanes (not shown) and the first turbo-expander 114 adjustable inlet nozzles (not shown). Thus, the smaller the proportion of liquid nitrogen product that is produced the lower the pressure to which the incoming air is compressed. Indeed, if desired, it is even possible to arrange the nitrogen generator such that no liquid nitrogen product is produced. In this case, the first turbo-expander 114 can be by-passed and the pressure to which the incoming air is raised drop to a little above that at which it is required in the fractionation column 116. The advantage of being able to turn up and down the proportion of liquid nitrogen product, typically in a range of 0 to 10% of the total nitrogen product, is an important one as it enables the plant operator to set the liquid nitrogen production according to demand and thereby enables a market for the liquid nitrogen product to be developed over a period of time without having to operate the plant uneconomically while the market is being developed.

[0030] Various changes, modifications and additions to the nitrogen generator shown in FIG. 1 may be made. Some of these are detailed below. One possible modification is to compress the incoming air in a plurality of compressors separate from one another. If a plurality of compressors is used, it is possible to effect the pre purification of the air at a location intermediate of the compressors, though such a practice is not preferred. Another modification that can be made is to omit the expansion or throttling valve 118 and its associated inlet to the fractionation column 116 and to cause all the purified air to flow through the first turbo-expander 114. Since it is desirable to maintain a balance between liquid flowing into the fractionation column 116 and liquid flowing out of it, the nitrogen generator is arranged such that the expanded air stream flowing from the first turbo-expander 114 and/or the recycle air stream from the compressor 138 contain an appropriate amount of liquid at their inlet to the fractionation column 116. Such an arrangement has the advantage of facilitating adjustment of the proportion of the nitrogen product that is produced in liquid state.

[0031] Although the external work performed by the first turbo-expander 114 and the second turbo-expander 128 may simply be the dissipation of energy through a braking mechanism, it is preferred to recover useful work from these machines. Accordingly, the second turbo-expander 128 may be coupled to an electrical generator (not shown) or may be employed to drive the recycle compressor 138. Thus, the rotor of the second turbo-expander 128 may be mounted on the same shaft (not shown) as the rotor or rotors of the compressor 138. In general, it is typically found that more work can be generated by expansion of the oxygen-enriched air stream exiting the first condenser 126 than is needed to drive the recycle compressor 138. In such circumstances, it is possible to replace the single second turbo-expander 128 shown in FIG. 1 of the drawings with two such machines in parallel with one another. One of the machines may receive that proportion of the oxygen-enriched air from the condenser 126 the expansion of which provides just the necessary amount of work to drive the recycle compressor 138, and the other machine can receive the excess oxygen-enriched air, typically being coupled to a generator (not shown) of electrical power.

[0032] Another modification can be made to the plant shown in FIG. 1 is that the first condenser 126 and second condenser 134 can be combined into a single unit

[0033] Another feature of the plant shown in FIG. 1 is that a part of the refrigeration for the condensers 126 and 134 is provided by a stream of liquid air taken from an intermediate region of the fractionation column through the outlet 130. As a result, the stream has a lower mole fraction of oxygen than the oxygen-enriched liquid air stream withdrawn from the outlet 120 at the bottom of the fractionation column 116. Therefore, it can be employed at a higher pressure in the condenser 134 than the pressure at which the stream withdrawn from the bottom outlet 120 is used in the first condensor 126. This in turn has the consequence that less work of compression needs to be done in the recycle compressor 138 than were the feed to it come from a stream derived from the bottom of the fractionation column 116. Typically, if the pressure at the top of the fractionation column 116 is in the order of 11 bar, the pressure at the inlet to the recycle compressor 138 is in the order of 7 bar.

[0034] There is some flexibility when designing the plant shown in FIG. 1 to choose the composition of the liquid fraction collected at the bottom of the fraction column 116. The greater the mole fraction of oxygen in this liquid, the smaller the proportion of nitrogen in the incoming air that is vented to the atmosphere and hence the greater the recovery of nitrogen by the method according to the invention. On the other hand, the greater the mole fraction of oxygen in the bottom fraction the greater is the amount of work of separation that needs to be done. Further, the greater the mole fraction of oxygen in the bottom liquid the higher is the pressure at which it has to be employed in condensing the top nitrogen fraction and hence the smaller the amount of refrigeration that can be generated by the second turbo-expander 128. In practice, a reasonable compromise between these conflicting criteria can be reached if the bottom fraction contains an amount of oxygen between 40 and 50% by volume. This equates to a nitrogen recovery in the range 61-74% if 10% of the nitrogen product is produced in liquid state and the remainder as gas at 11 bara. The required air pressure at the air compressor varies between 17-25 bar to produce the product mix above, depending on the chosen process conditions. The total energy consumption per unit of nitrogen produced is in the order of 0.23 kWh/Nm³, and remains close to this value whether the chosen method is high air pressure/high recovery, or low air pressure lower recovery. The proportion of the product produced as liquid may be increased at least to 20% by raising the air inlet pressure so as to increase the refrigeration capacity of the turbines.

[0035] Referring now to FIG. 2 of the drawings, there is shown a double column nitrogen generator according to the invention. The refrigeration system employed by this nitrogen generator is analogous to that of the single column nitrogen generator shown in FIG. 1. The ensuing description shall concentrate more on the features of the double nitrogen generator that have no analogous counterparts in the single column nitrogen generator shown in FIG. 1.

[0036] Referring again to FIG. 2, air is compressed in a compressor 202, cooled in an aftercooler 204 and purified in a purification unit 206. The purified air slows from the purification unit 206 into a main heat exchanger 208, entering the main heat exchanger 208 at its warm end 210. Most of the air is withdrawn from the main heat exchanger 208 at its immediate region thereof and is expanded with the performance of external work in a first expansion turbine or turbo-expander 214. The resulting expanded air flow is reintroduced into the main heat exchanger 208 for further cooling and is cooled to a temperature suitable for its separation by rectification therein. The expanded air stream exits the main heat exchanger 208 from its cold end 212 and is introduced through a bottom inlet 216 into a double rectification column 218. The double rectification column 218 comprises a higher pressure rectification column 220, a lower pressure rectification column 222, and a condenser-reboiler 224 which thermally links the top of the higher pressure rectification column 220 to the bottom of the lower pressure rectification column 222. The bottom inlet 216 communicates with the bottom of the higher pressure rectification column 220. The air that is introduced into the higher pressure rectification column 220 through the bottom inlet 216 is separated therein into a top product nitrogen vapour fraction and a bottom oxygen-enriched liquid air fraction. The bottom fraction typically contains from 35 to 40 mole % of oxygen. The higher pressure rectification column 220 contains liquid-vapour contact devices in order effect mass exchange between ascending vapour and descending liquid. Typically, the devices comprise structured packing.

[0037] The bottom oxygen-enriched liquid fraction obtained in the higher pressure rectification column 220 is used as a feed to the lower pressure rectification column 222. A stream of the bottom fraction is withdrawn through an outlet 226, is sub-cooled in a heat exchanger 228, is passed through an expansion or throttling valve 230 and is introduced into an intermediate mass exchange region of the lower pressure rectification column 222. A second feed stream for the lower pressure rectification column 222 is provided by that part of the purified air which does not flow to the first turbo-expander 214. This residual purified air exits the main heat exchanger 208 through its cold end 212, is sub-cooled by passage through the heat exchanger 228, is expanded by passage through an expansion or throttling valve 232 and is introduced into an intermediate mass exchange region of the lower pressure rectification column 222.

[0038] The air is separated in the lower pressure rectification column into a bottom product oxygen-enriched liquid air fraction typically containing in the order of 505 by volume of oxygen and a top nitrogen vapour fraction. Ascending vapour is intimately contacted with descending liquid in the column 222 by means of liquid-vapour contact devices (not shown) such as structured packing. The upward flow of vapour in the column 222 is created by operation of the condenser-reboiler 224 to reboil the liquid fraction collecting at the bottom of the column 222. The reboiling is effected by indirect heat exchange of the liquid with nitrogen vapour separated in the higher pressure rectification column 220. This vapour is thus condensed in the condenser-reboiler 224, the resulting condensate being employed as reflux in the higher pressure rectification column 220. The nitrogen fraction separated in the lower pressure rectification column 222 is condensed in a condenser 233. Cooling for the condenser 233 is provided by means of a stream of the bottom oxygen-enriched liquid air fraction formed in the lower pressure rectification column 222. Thus, a stream of this fraction is withdrawn through an outlet 234 and is sub-cooled in the heat exchanger 228 and is then expanded through an expansion or throttling valve 236. The resulting expanded bottom fraction is introduced into the condenser 232 and is vaporised by indirect heat exchange with condensing nitrogen. The resulting oxygen-enriched air vapour is returned through the heat exchanger 228, thereby providing the necessary cooling for it, is warmed by passage through the main heat exchanger 208 from its cold end 212 to an intermediate region thereof, and is expanded with the performance of work in a second turbo-expander 238. The resulting expanded oxygen-enriched air stream is returned through the main heat exchanger 208 from its cold end 212 to its warm end 210. The resulting warmed oxygen-enriched air stream may be vented to the atmosphere.

[0039] Preferably, all the nitrogen product is taken from the higher pressure rectification column 220. Accordingly, the vapour stream is withdrawn from the top of the higher pressure rectification column 220 through a conduit 240 and is warmed to approximately ambient temperature by passage through the main heat exchanger 208 from its cold end 212 to its warm end 210. The resulting warmed nitrogen stream may be taken as product.

[0040] In addition, a liquid nitrogen product is preferably taken from the nitrogen that is condensed in the condenser-reboiler 224. An outlet 242 is provided for this purpose. In order to enable the nitrogen products to be taken from the higher pressure rectification column 220, a proportion of the nitrogen condensed in the condenser 232 is pumped by a pump 244 through the heat exchanger 228 from its cold end to its warm end and is returned to an intermediate mass exchange region of the higher pressure rectification column 220 through an inlet 246. The number of theoretical trays in the higher pressure rectification column 220 above the level of the inlet 246 depends on the desired purity of the nitrogen product and on the purity of the nitrogen that is separated in the lower pressure rectification column 222. That part of the liquid nitrogen that is condensed in the condenser 32 which is not pumped back to the higher pressure rectification column 220 is employed as reflux in the lower pressure rectification column 222.

[0041] Typically, in operation of the nitrogen generator shown in FIG. 2, the higher pressure rectification is operated at a pressure in the order of 11 bar at its top and the lower pressure rectification column 222 at a pressure of about 7 bar at its top. The first turbo-expander 214 has an inlet pressure in the order of 16 bar when approximately 10% of the nitrogen product is produced in liquid state. The second turbo-expander 230 typically has an inlet pressure in the order of 3 bar, and an outlet pressure in the order of 1.2 bar. The nitrogen generator shown in FIG. 2 may typically produce up to 10% of its nitrogen product in liquid state, obtain a nitrogen recovery of 70% at a specific power of 0.22 kWh/Nm³. The proportion of the product produced as liquid may be increased up to 30%, if the inlet air pressure is increased in order to increase the turbine cold production.

[0042] Analogously to the nitrogen generator described with reference to FIG. 1, the proportion of nitrogen product that is produced in liquid state can be varied by varying the inlet pressure to the first turbo-expander 214. 

1. A method of generating nitrogen, wherein compressed air is separated at elevated pressure by fractionation to produce a gaseous nitrogen product fraction and an oxygen-enriched liquid product fraction containing less than 80 mole percent of oxygen, a gaseous nitrogen product is taken at elevated pressure from the top gaseous nitrogen product fraction, a stream of the product oxygen-enriched fraction is vaporised by indirect heat exchange with condensing nitrogen separated in the fractionation, a liquid nitrogen product is generated with the gaseous nitrogen product by condensing nitrogen separated in the fractionation, and refrigeration for the method is created (i) by expanding a stream of compressed air in a first expansion turbine with the performance of external work and (ii) by expanding at least part of the vaporised stream of the product oxygen-enriched fraction in a second expansion turbine with the performance of external work, characterised in that the said stream of compressed air enters the first expansion turbine at a pressure substantially in excess of the pressure at which the gaseous nitrogen product is taken but leaves the first expansion turbine at essentially the gaseous nitrogen product pressure, and in that at least part of the air separated in the said fractionation column is supplied from the first expansion turbine:
 2. The method according to claim 1, in which the fractionation is performed in a single fractionation column.
 3. The method according to claim 2, in which a stream of a liquid mixture of nitrogen and oxygen is withdrawn from the bottom or an intermediate mass exchange region of the single column, is reduced in pressure, is heat exchanged with condensing nitrogen separated in the single column, and is vaporised thereby, and a stream of the resulting vapour is compressed in a recycle compressor and is returned to a bottom mass exchange region of the single column.
 4. The method according to claim 1 in which the fractionation is performed in a double fractionation column comprising a higher pressure column and a lower pressure column.
 5. The method according to claim 4 in which all the gaseous nitrogen product is taken from the higher pressure column.
 6. The method according to claim 4, in which liquid nitrogen separated in the lower pressure column is pumped to the higher pressure column.
 7. The method according to claim 1, in which the proportion of nitrogen product produced in liquid state is varied by varying the inlet pressure of the first expansion turbine.
 8. The method according to claim 1, in which at least 85% by volume of the gaseous air to be separated by fractionation flows through the first expansion turbine.
 9. A nitrogen generator for performing the method claimed in claim 1, comprising a fractionation column operable to produce an elevated pressure gaseous nitrogen product and an oxygen-enriched air product containing less than 80 mole % of oxygen, the nitrogen generator also being able to co-generate a liquid nitrogen product, a first expansion turbine for expanding with the performance of external work a first stream of compressed air, a nitrogen condenser for condensing nitrogen separated in the fractionation column by indirect heat exchange with a vaporising stream of the product oxygen-enriched liquid air, and a second expansion turbine for expanding a stream of the vaporised product oxygen-enriched air with the performance of external work, characterised in that the outlet of the first expansion turbine communicates with the fractionation column.
 10. The nitrogen generator according to claim 9, in which the fractionation column is a single fractionation column
 11. The nitrogen generator according to claim 10, additionally comprising a further condenser for condensing nitrogen, the further condenser having passages therethrough for a stream of liquid comprising a mixture of oxygen and nitrogen taken from the fractionation column, the said passages communicating at their outlet end with a recycled compressor for returning the vaporised mixture to the bottom of the fractionation column.
 12. The nitrogen generator according to claim 9, wherein the fractionation column is a double fractionation column comprising a higher pressure column and a lower pressure column.
 13. The nitrogen generator according to claim 12, additionally including a pump for sending condensed nitrogen separated in the lower pressure column to the higher pressure column. 